Ported can go much deeper, but it costs to go below 20 hz, and not all ported subs really play that deep. Read some reviews of whatever sub youre interested in before deciding,

Cheap chinese anything comes down to R&D, parts, warranty, and quality control. There’s also misleading information, but that applies to well known brands too.

It’s not magic. Better chi-fi copy, or use a basic design, with maybe some tweaks, or cost cutting measures. Some items even come from the same factories. Having those factories, and cheap labor keeps costs rock bottom. More savings comes from marketing, or the lack of it.